This invention relates to a television transmitter.
Conventional modern television transmitters carry out vision modulation at an intermediate frequency (i.f.) which is typically 38.9 MHz. This modulation process is generally combined with filtering to provide the "vestigial side band" characteristic adopted by virtually all terrestrial T.V. broadcasting.
A television receiver takes analogue amplitude modulated vision signals which provide the picture information and synchronising information to ensure that the displayed luminance and chrominance information appear in the right place on the television screen. The picture and synchronisation information are separate at some points in the chain between the televison camera and the television receiver, but appear as a composite wave form generally when being transferred from one place to another. A standard video waveform as presented to a transmitter in a U.K. P.A.L. system, usually as a one volt signal, is illustrated in FIG. 1, which shows a typical line where the luminance level varies between black and white a number of times between the left and right sides of the picture display. For convenience, a test wave form will be considered, being a line saw tooth, changing linearly between black at the left of the screen and white at the right, as shown in FIG. 2. For the sake of simplicity, FIG. 2 ignores the chrominance information and the vertical synchronisation. The radio frequency envelope broadcast by a television transmitter, corresponding to the test waveform shown in FIG. 2, is illustrated by FIG. 3.
In a conventional transmitter, the radio frequency envelope of this form is amplified to the necessary power level. To avoid distortion of the modulation envelope, linear amplifiers are required. Such amplifiers are typically class AB to attain as high an efficiency as possible. The amplifier must be capable of handling the sync pulse power, which it will be seen from FIG. 3 represents a fairly short pulse of much higher power than the rest of the signal. Because of this, some efficiency in amplification is lost for the greater part of the signal in allowing for the sync. pulse.
In UK Published Patent Application No. 2223626 there is disclosed and claimed a television transmitter in which separate picture signal and synchronisation pulse modulation of the r.f. carrier is followed by separate amplification for the modulated carrier signals, which are then combined in a signal combining device as specified in the claims in said application. Thus, when such a transmitter includes solid state amplification both of the separately modulated signals required a multi-way splitter so that the signal can be divided equally between a number of amplifiers in parallel, each solid state amplifier being of limited power.